Culture is shaped by its regulations and resources. Also media may be a factor, with movies and video games. People say that the video games don’t have much impact, but we can clearly see that the U.S. Killers clearly prefer to shoot a mass amount of victims rather than blow them up.
It’s certainly a lot easier to blow people up by tossing a grenade rather than using an automatic rifle. You can drop and run, like the Boston Marathon bombing. They got their message across without exposing themselves immediately. The shooters know they are going to be discovered immediately, but something tells them that it’s ok.
Maybe it’s easier to train for mass shootings than explosives. They all have to do their research before they set out on their mission. Should I blow up this crowd of people or shoot them all down? Maybe it depends on the event. People move around a lot and it can be hard to blow them all up effectively. Picking off unarmed victims may be more rewarding.
There was the Olympic bombing in the US with Eric Rudolph. Why couldn’t he shoot people up instead? He made a decision that bombs were a better idea. Then there was Ted Kazinsky. He preferred bombs over shooting as well. Maybe the choice between bombing and shooting is pretty even in the US. I don’t know.